BULL AND MUSK (BIENNIAL)
THISTLE CONTROL IN
PERENNIAL GRASS CROPS

GENERAL
CHARACTERISTICS OF BIENNIAL THISTLES

Musk thistle and bull thistle are biennials causing major problems
in perennial grass crops including pastures, roadsides, conservation
reserve and fence rows. Biennials require portions of two growing
seasons to reproduce. They grow from seed the first season as
a rosette (a taproot with a cluster of leaves on the soil surface).
The rosette overwinters and cold causes the rosette to send up
a flowering stalk the next season and produce seed. Once seeds
mature, the plant dies. Destruction of rosettes prior to flowering
is an effective means of preventing seed formation and subsequent
spread.

Management of pastures infested with biennial thistle requires
special consideration. Since biennial thistles reestablish from
seed which is dispersed by wind, it is helpful to prevent seed
formation adjacent to pastures. Also it may take two or more years
of excellent control before seeds are reduced to the point that
allows for legume reestablishment for pasture improvement. One
year of poor thistle control will result in having to start the
control program over.

Most of the herbicides used for control of bull and musk thistle
also kill pasture legumes. Spot spraying individual plants or
patches rather than broadcast spraying the entire pasture also
spares the legumes.

Musk thistle normally initiates flower stalks in early
May and reaches full flower in early June. Seed production is
prolific and usually completed in mid to late June. Rosettes reestablish
nearly any time during the growing season. Some rosettes may be
three or four feet in diameter by late fall. Musk thistle is extremely
aggressive.

Bull thistle normally initiates flower shoots in July
and reaches full flower in August. Seed production is usually
completed in late summer. Rosettes reestablish during summer and
early fall.

POSTEMERGENCE
HERBICIDES FOR CONTROL OF BIENNIAL THISTLES

The best time to treat biennial thistles with herbicides
is in late fall or early spring when the rosettes are present
but before flowering stalks are initiated. Musk thistle and bull
thistle plants with seed stalks are more difficult to kill than
the rosettes.Thistle rosettes need to be treated when
they are actively growing and not under drought stress. The younger
the rosette, the more susceptible it is to herbicide.

One properly timed treatment per year should prevent seed formation.
Several herbicides provide good to excellent control of thistle
rosettes. Fall treatments should be made late enough to kill all
rosettes germinated before winter. Late germinating rosettes that
establish after early fall herbicide applications could flower
the next growing season. Early spring treatments should kill all
overwintering rosettes and those rosettes germinating later (spring
and summer) should not produce seed until the following year.

STINGER (clopyralid) 1/3 to 2/3 pt./acre. Spot treatment
only or when Canada thistle is present. Forage legumes will also
be killed. Legumes cannot be reseeded into treated areas for 12
months following treatment

Plateau (imazapic) 8 to 12 oz./acre for Conservation Reserve,
and wild flower establishment and other non-cropland only uses.
Doses of imazapic PLATEAU that control thistle tend to severely
injure tall fescue.

TORDON (picloram) long soil-residual selective control
in permanent pastures where brush control is also desired.